1. Field of the Invention
This invention relates to generally to the conversion of high voltage alternating current (AC) to low voltage direct current (DC) and, more specifically, to an apparatus and a method for converting high voltage AC to low voltage DC without the use of transformers, large capacitive coupling circuits or high voltage linear regulators and with improved efficiency.
2. Description of the Prior Art
There are devices such as consumer appliances and electronics, i.e. refrigerators, washing machines, dishwashers, microwave ovens, etc., which require high voltage AC power and low voltage DC power. The low voltage DC requirement is for powering analog and digital control circuitry, display indicators such as Light Emitting Diodes and other low power devices.
The prior art identifies attempts to provide AC to DC conversion in three principal categories: the transformer approach, the high voltage linear regulator approach and the high voltage capacitive coupling approach. Each of these three approaches has limitations which are discussed below.
Referring to FIG. 1, the transformer approach with full wave rectifier is illustrated. The step down transformer will drop the input voltage, which is typically 110-120 VAC for devices operating in the U.S. and Canada and typically 220-240 VAC for devices operating in Europe and elsewhere in the world, to a low voltage in the range of 5-24 VAC, depending on the application. After step down, the sinusoidal AC input is then rectified by a full wave rectifier, i.e. diodes D1, D2, D3 and D4. The capacitors C1 and C2 combine with the linear regulator to provide a stable DC output voltage VOUT.
The disadvantage to this approach, and to all transformer approaches, is the prohibitive cost, size, weight and power consumption of step down transforms. Furthermore, the approach of FIG. 1 also requires a four diode bridge rectifier.
Referring to FIG. 2, a step down transformer is used in conjunction with a half wave rectifier. The transformer provides a low voltage AC component as in FIG. 1. However, in the case of FIG. 2, a single diode D1 is used to form the half wave rectifier. The capacitors C1 and C2 with the linear regulator provide a stable DC output voltage. Although this approach uses only one diode, as compared to four diodes in FIG. 1, the capacitor C1 must be significantly larger than its counterpart in the full wave rectifier configuration to compensate for the half wave rectification. Thus, the disadvantage to this approach, in addition to the step down transformer, is the size of the capacitor C1.
Referring to FIG. 3, another configuration of the transformer approach is illustrated using a center tap transformer and a full wave rectifier comprised of diodes D1 and D2. The center tapped transformer, while permitting a two diode full wave rectifier, adds complexity and therefore cost to the configuration.
Referring to FIG. 4, the high voltage linear regulator approach is illustrated. In this approach, the bulky and costly step down transformer is eliminated from the circuit. The high voltage AC input is rectified by the full wave rectifier, diodes D1, D2, D3 and D4 and stored by capacitor C1. The high voltage linear regulator reduces the high DC voltage to a low DC output voltage, typically in a range of 5-24 VDC. Capacitor C2 provides a filter for the DC output voltage. The disadvantage of the high voltage linear regulator approach is excessive power dissipation caused by the storage of high voltages on capacitor C1.
Referring to FIG. 5, the high voltage capacitive coupling approach is illustrated. Once again the step down transformer is eliminated. Capacitor C1 couples the AC component to the full wave rectifier, i.e. diodes D1, D2, D3 and D4, across resistor R1. The zener diode Z1 limits the output of the full wave rectifier to the desired low voltage DC output. Capacitor C2 provides a filter for the DC output voltage. Although this approach reduces the power consumption over the approach shown in FIG. 4, the size of capacitor C1 is prohibitively large. Furthermore, it also suffers from high no-load power consumption.
Therefore, a need existed to provide an improved apparatus and method for converting high voltage AC to low voltage DC. The improved apparatus and method for converting high voltage AC to low voltage DC must overcome the problems associated with prior art devices and methods. The improved apparatus and method must convert high voltage AC to low voltage DC without the use of transformers, large capacitive coupling circuits or high voltage linear regulators and with improved efficiency.